Coding guidelines -- using directives

Three kinds of using directives are available:
* Reference: using <namespace> -- namespace can be omitted when referencing types.
* Alias: using <alias> = <namespace|type> -- alias can be used in place of namespace
or type.
* Static: using static <type> -- all static members of type are accessible without
having to specify the type name.

The following conventions apply to using directives in the Qt VS Tools code:
* using directives are grouped by root namespace, and ordered alphabetically within each group.
* Directives that reference external namespaces are located at the start of the source file,
before the local namespace declaration.
* The order of external namespace groups is as follows:
1. System* namespaces.
2. Microsoft* namespaces.
3. EnvDTE* namespaces.
4. All other external namespaces.
* Directives referencing in-solution namespaces (i.e. QtVsTools* or QtVsTest* namespaces) are
nested within the local namespace block.
* In-solution reference directives will use an abbreviated form of the namespace, whenever
possible.
* Alias and static directives can be specified either at top-level or nested in the local
namespace.
* Alias directives are specified after all reference directives.
* Static directives are specified after all reference directives and all alias directives.
* Optionally, directives of different kinds can be separated with an empty line.

Example

using System;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text.RegularExpressions;
using Microsoft.VisualStudio;
using Microsoft.VisualStudio.Shell;
using Microsoft.VisualStudio.VCProjectEngine;
using EnvDTE;
using EnvDTE80;

using Task = System.Threading.Tasks.Task;

namespace QtVsTools
{
    using Core;
    using QtMsBuild;

    using RegExprParser = SyntaxAnalysis.RegExpr.Parser;

    using static SyntaxAnalysis.RegExpr;

    class ...